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as the eighteenth century, certain sorts of people were thought so prone to lie on the stand that they were not allowed to take the oath: children, the spouse of the accused, those with a financial interest in the outcome, slaves, and the defendant. It was only after the American Civil War that U.S. courts finally allowed defendants to be sworn in when they took the stand in their own defense (and then mainly to avoid the accusation of hypocrisy as newly freed slaves were allowed to testify under oath). Only then, with all parties sworn in, did jurors have full freedom to weigh the testimony of all witnesses in an evenhanded manner, assessing their words and demeanor: the way they moved, blinked, blushed, sweated, or spoke. The institution of the jury may be 1,000 years old, but only in the past century have jurors acquired an unfettered license to distinguish truth-tellers from liars.
    Yet over this same period, the justice system increasingly sought to corroborate human testimony with circumstantial evidence beyond the power of human beings to dissemble. Because lay juries and magistrates were themselves unable to assess such evidence, the courts increasingly turned to expert witnesses. The courts already drew on the testimony of skilled practitioners of specialized crafts—surveyors, physicians, trades workers—regarding technical matters of which they had direct knowledge. But only during the eighteenth century were adversarial lawyers in England permitted to call experts who extrapolated from special experimentation using general scientific principles to give their opinion regarding the case at hand. With the growing reach and authority of science in the nineteenth century, reformers hoped these experts would underwrite the reliability of a justice system otherwise in the hands of a lay jury.
    The problem was that adversarial lawyers proved adept at finding adversarial experts to make diametrically opposed arguments. By the latter half of the nineteenth century, quarreling among experts in the courtroom had become a subject of general mockery. One legal aphorism of the period had it that there were three kinds of liars, "the common liar, the damned liar, and the scientific expert." It didn’t help that a vast array of practitioners of new sciences clamored for admission to the courtroom. How would the law decide whom to admit? The dream of certainty dies hard.
    In the early decades of the twentieth century a coterie of reform-minded Americans set out to inaugurate a new phase in the administration of justice: the scientific interrogation of accused persons and other witnesses. The time had come, these reformers proclaimed, for the law’s archaic methods of assessing human honesty to give way to the new science of psychology. Just as the discovery of X-rays now allowed expert radiologists to peer into a patient’s body—and, according to some physicians, perhaps the mind as well—so might these new instruments allow expert psychologists to peer into a witness’s body to infer whether the conscience was disturbed. The human body, suggested these investigators, could serve as a kind of circumstantial evidence for the mind.
     
    This challenge to the law first issued from the laboratory of Marston’s mentor, Hugo Münsterberg of Harvard. In the years before World War I, Münsterberg, a German émigré, founded "brass instrument" psychology. Lured to Harvard by William James, but despised by him in later years, Münsterberg was the first scientist to lay out the rationale for a science of lie detection. The seventeenth-century polymath Gottfried Leibniz had famously asked whether we would understand human consciousness any better if we could visit the machinery of our minds as if taking a factory tour. At the turn of the twentieth century Münsterberg created such a factory over Harvard Yard: a lab filled with students transforming their inner feelings into the kind of public phenomena we call science.
    Münsterberg